1. Field of the Invention
The present invention relates to a method and an apparatus for decomposing an organic halogen-containing compound, and in particular to a technique for efficiently decomposing an organic halogen-containing compound by ultraviolet ray irradiation.
2. Description of the Prior Art
Halogen-containing organic compounds such as trichloroethylene, polychlorobiphenyl (PCB), various Freons ("Freon" is a trademark for certain fluorocarbon compounds manufactured by Du Pont) and the like have been widely used in the field of chemical industry and also in other industrial fields. Of these organic compounds, chlorine-containing compounds, e.g. trichloroethylene, PCB, etc. have been already confirmed as being poisonous to humans. Fluorine-containing compounds have not as yet been found to be toxic to humans. However, recent warnings have been issued that the fluorine-containing compounds seem to destroy the ozonosphere to adversely affect the global atmosphere. In such circumstances, the public interest has been increasingly attracted toward methods of decomposing halogen-containing compounds, and various methods have been researched in order to lessen the harm from such compounds.
Specifically, techniques such as a catalytic method, an ultraviolet irradiation method, a sodium decomposition method, etc. are now under development.
In the catalytic method, decomposition of the organic halogen-containing compound is carried out with a catalyst in a liquid phase by consuming oxygen contained in the liquid phase. This method is easily practiced and does not need much driving energy, therefore it is economically advantageous. However, the decomposition efficiency is rather low due to low concentration of the oxygen in the liquid phase. Moreover, halide products such as hydrogen fluoride, hydrogen chloride, etc. which are generated by the decomposition reaction cause a problem by rapidly reducing the activity of the catalyst.
In the ultraviolet radiation method, an organic halogen-containing compound in the gaseous state is decomposed by irradiating with ultraviolet rays. In this method, the halide products produced in the decomposition reaction hinder further process of the decomposition reaction. As a result, it becomes difficult to complete the decomposition reaction of the organic halogen-containing compounds by ultraviolet irradiation only, and the decomposition efficiency remains low. Moreover, this method involves the risk of damaging the apparatus with corrosive halide gases at a higher frequency than in conventional methods. In addition, this method tends to require large-scale apparatus. Therefore, this method is disadvantageous, unless the above-described problem with decomposition efficiency is solved.
In the sodium decomposition method, the organic halogen-containing compounds are decomposed with sodium at a high decomposition efficiency. However, since this method consumes a large amount of the sodium material, it is economically disadvantageous. Moreover, in the apparatus for carrying out this method, it is necessary to provide much equipment for ensuring operational safety during handling of the sodium material. As a result, this apparatus requires a large space for installation.
Of the above-described decomposition methods, the ultraviolet radiation method has been recently improved and suggested for decomposition of chlorine-containing compounds. In this improved method, for increasing the velocity of decomposition reaction, the chlorine-containing compounds are dissolved in an alcohol solvent which contains an alkaline metal material, and ultraviolet rays are used to irradiate that solution. In this reaction system, the chloride compound, i.e. hydrogen chloride produced by the decomposition reaction, forms a salt with the alkaline metal in the solution, and the salt is precipitated and removed from the solution. Accordingly, hindrance of the decomposition reaction by the decomposed product can be prevented.
However, in the above-described ultraviolet irradiation method, the alcohol molecules of the solvent may be transferred to the corresponding aldehyde or ketone form in the reaction system by ultraviolet radiation, and the aldehyde, ketone and the like are polymerized in the presence of the alkaline material in accordance with the aldol condensation reaction. This reaction badly affects the reaction system to make it difficult to repeat the decomposition operation. Therefore, this method is poor in practice.
As described above, each of the conventional decomposition methods has been associated with problems hindering the actual use, and no technique for actual use is as yet known.